This invention relates to an alkali-free glass and, in particular, to an alkali-free glass for use as a light transparent glass substrate for a liquid crystal display and the like as well as a method of producing the same.
Conventionally, an alkali-free glass has been used as the light transparent glass substrate for the liquid crystal display and the like. The alkali-free glass for use in such a display is required to be free from bubbles such as blisters and seeds resulting in a display defect, in addition to various characteristics such as heat resistance and chemical resistance.
To meet such a demand, various kinds of alkali-free glasses have been proposed. U.S. Pat. No. 4,994,415 discloses an SiO2xe2x80x94Al2O3xe2x80x94B2O3xe2x80x94CaOxe2x80x94BaO as one of the alkali-free glasses.
In order to obtain a glass without bubbles, it is important to select a fining agent capable of generating a fining gas both in a relatively low-temperature range in which batch decomposition and degassing is started and in a relatively high-temperature range in which fining and homogenization of a glass are caused. Specifically, the fining agent serves to expel a gas generating during the batch decomposition and degassing of a glass material, and to enlarge very small bubbles remaining in the glass melt during fining and homogenization processes, whereby enlarged bubbles float up in the glass melt and leave the glass melt.
In the meanwhile, the alkali-free glass for use as a glass substrate for a liquid crystal display is high in viscosity of the glass melt so that a melting process is carried out at a high temperature in comparison with a glass containing an alkali component. In the alkali-free glass of the type, the batch decomposition and degassing generally occurs at 1200-1300xc2x0 C. while the fining and the homogenization are performed at a high temperature of 1400xc2x0 C. or more. Under the circumstances, the fining agent is required to produce the fining gas in a wide temperature range (on the order of 1200-1600xc2x0 C.). Presently, As2O3 is widely used as the fining agent.
However, As2O3 is highly toxic and may possibly cause environmental pollution during a manufacturing process of the glass and during disposal of a waste glass. In this reason, the use of As2O3 is being limited.
It is an object of the present invention to provide an alkali-free glass which does not use As2O3 as a fining agent and which is free from bubbles resulting in a display defect, and to provide a method of producing the same.
As a result of various experiments, the present inventors have found the fact that above-mentioned object is achieved by using a combination of Sb2O3 and at least one of SnO2 and chloride, as a fining agent instead of As2O3.
According to the present invention, there is provided an alkali-free glass essentially consisting of basic elements of 40-70% SiO2, 6-25% Al2O3, 5-20% B2O3, 0-10% MgO, 0-15% CaO, 0-30% BaO, 0-10% SrO, and 0-10% ZnO, and a fining agent of a combination of 0.05-3% Sb2O3 and at least one of 0.05-2% SnO2 and 0.005-1% Cl2, on the base of the weight percent.
According to the present invention, there is also provided a method of producing an alkali-free glass having a basic composition essentially consisting by weight of 40-70% SiO2, 6-25% Al2O3, 5-20% B2O3, 0-10% MgO, 0-15% CaO, 0-30% BaO, 0-10% SrO, and 0-10% ZnO, by preparing a glass batch having the basic composition free from alkali metal oxide, melting the glass batch, and forming the glass melt, which is characterized by adding a combination of 0.05-3 wt % Sb2O3 and at least one of 0.05-2 wt % SnO2 and 0.01-2 wt % chloride in terms of Cl2 as a fining agent into the glass batch.
Sb2O3 and SnO2 used in the present invention generate a large amount of fining gas (oxygen gas) as a result of chemical reaction following variation in valence of Sb ions and Sn ions. Specifically, Sb2O3 (trivalent) is at first changed to Sb2O5 (pentavalent) in a low-temperature range on the order of several hundred degree in xc2x0 C. and then returns to Sb2O3 (trivalent) around 1200-1300xc2x0 C. At this time, a large amount of fining gas is released. Likewise, when SnO2 (quadrivalent) is changed to SnO (bivalent) at 1400xc2x0 C. or more, a large amount of fining gas is released. On the other hand, chloride is decomposed and volatilized in a temperature range not lower than 1200xc2x0 C. to generate the fining gas (for example, chroline gas). Particularly, decomposition and volatilization are very active at a high-temperature range not lower than 1400xc2x0 C. to generate a large amount of fining gas.
Therefore, the use of a combination of Sb2O3 and at least one of SnO2 and chloride as the fining agent provides a high fining effect at a wide temperature range from a comparatively low temperature for the batch decomposition and degassing to a high temperature for fining and homogenizing. Thus, it is possible to provide the alkali-free glass without bubbles resulting in a display defect.
Next, description will be made about a method of producing of an alkali-free glass according to the present invention.
At first, a raw glass mixture or a glass batch is prepared so as to obtain a glass having the above-mentioned composition. Description will hereafter be made about the content of each component in the composition and the reason why the content is so defined.
SiO2 is a component serving as a network of the glass. The content of SiO2 is 40-70%, preferably, 45-65%. When the content is less than 40%, chemical resistance is degraded and a strain point of the glass is lowered so that heat resistance is degraded. When the content is more than 70%, high-temperature viscosity is increased so that the meltability is deteriorated and that devitrified substances of cristobalite readily precipitate.
Al2O3 is a component to improve the heat resistance and devitrification resistance of the glass. The content of Al2O3 is 6-25%, preferably, 10-20%. When the content of Al2O3 is less than 6%, the devitrification temperature remarkably rises so that the devitrification is readily caused to occur in the glass. When the content is more than 25%, acid resistance, more particularly, buffered-hydrofluoric-acid resistance is degraded so that the cloudness is readily caused to occur on the surface of a glass substrate.
B2O3 is a component serving as a flux to lower the viscosity and to facilitate melting of the glass. The content of B2O3 is 5-20%, preferably, 6-15%. When the content of B2O3 is less than 5%, the effect as the flux is insufficient. When the content is more than 20%, the hydrochloric acid resistance is degraded and the strain point is lowered so that the heat resistance is degraded.
MgO is a component to decreasing the high-temperature viscosity without lowering the strain point so as to facilitate melting of the glass. The content of MgO is 0-10%, preferably, 0-7%. When the content of MgO is more than 10%, the buffered-hydrofluoric-acid resistance of the glass is seriously degraded.
CaO has a function similar to MgO. The content of CaO is 0-15%, preferably, 0-10%. When the content of CaO is more than 15%, the buffered-hydrofluoric-acid resistance of the glass is seriously degraded.
BaO is a component to improve the chemical resistance and the devitrification resistance of the glass. The content of BaO is 0-30%, preferably, 0-20%. When the content of BaO is more than 30%, the strain point is lowered so that the heat resistance is degraded.
SrO has an effect similar to BaO. The content of SrO is 0-10%, preferably, 0-7%. The content of SrO more than 10% is unfavorable because the devitrification is increased.
ZnO is a component to improve the buffered-hydrofluoric-acid resistance and the devitrification resistance. The content is 0-10%, preferably, 0-7%. When the content of ZnO is more than 10%, the glass tends to be devitrified and the strain point is lowered so that the heat resistance can not be assured.
When the total content of MgO, CaO, BaO, SrO, and ZnO is less than 5%, the high-temperature viscosity is increased so that the meltability is degraded and the glass is readily devitrified. The total content more than 30% is unfavorable because the heat resistance and the acid resistance are degraded.
In addition to the above-described components, it is possible to add ZrO2, TiO2, Fe2O3, and the like up to 5% in total.
Next, a combination of Sb2O3 and at least one of SnO2 and chloride is added as a fining agent to the glass batch. As a material for chloride, BaCl2, CaCl2, and the like can be used. As a material for Sb2O3, a pentavalent Sb compound such as Sb2O5 can be used instead of Sb2O3. The amounts of Sb2O3 and SnO2 to be added is 0.05-3 wt % and 0.05-2 wt %, respectively, with respect to the raw glass mixture as 100 wt %. On the other hand, the amount of chloride is 0.01-2 wt % in terms of Cl2, that is, an amount for producing 0.01-2 wt % Cl2 by decomposition of the chloride. When the content of Sb2O3 is less than 0.05%, it is difficult to expel the gas generating during the batch decomposition and degassing. When the content of SnO2 is less than 0.05% and the chloride is less than 0.01% in terms of Cl2, it is difficult to remove the bubbles remaining in the glass melt during the fining and homogenizing. On the other hand, when the content of SnO2 is more than 2% and the content of Sb2O3 is more than 3%, the glass tends to be devitrified. Furthermore, when the chloride is more than 2% in terms of Cl2, the amount of volatilization is excessive so that the glass tends to be deteriorated. It is possible to use only one of SnO2 and chloride. However, the use of both of them in combination is favorable because the excellent fining effect is obtained.
Then, the glass batch thus prepared is melted. When the glass batch is heated, at first the batch decomposition and degassing occurs. At this time, the oxygen gas generates as a result of chemical reaction following the variation in valence of Sb2O3. Therefore, the gas generating at the time of batch decomposition and degassing is expelled from the glass melt. Furthermore, during the fining and homogenizing at a higher temperature, the oxygen gas generates as a result of chemical reaction following the variation in valence of SnO2. In addition, chloride is decomposed and volatilized to produce the chlorine gas or chlorine compound gas. Thus, very small bubbles remaining in the glass melt are removed.
Then, the glass melt is formed into a desired shape. For use as a display, the glass melt is formed into a thin plate shape by the use of a fusion process, a downdraw process, a float process, a roll-out process, and the like.
Thus, it is possible to obtain the alkali-free glass of the present invention, essentially consisting of basic elements of 40-70% SiO2, 6-25% Al2O3, 5-20% B2O3, 0-10% MgO, 0-15% CaO, 0-30% BaO, 0-10% SrO, and 0-10% ZnO and a combination of 0.05-3% Sb2O3 and at least one of 0.05-2% SnO2 and 0.005-1% Cl2 as the fining agent.
In the following, the alkali-free glass of the present invention will be described in conjunction with specific examples.